Firefox 138 for developers

This article provides information about the changes in Firefox 138 that affect developers. Firefox 138 is the current Nightly version of Firefox and ships on April 29, 2025.

Changes for web developers

Developer Tools

HTML

Removals

CSS

Removals

JavaScript

Removals

SVG

Removals

HTTP

Removals

Security

Removals

APIs

DOM

Media, WebRTC, and Web Audio

Removals

WebAssembly

Removals

WebDriver conformance (WebDriver BiDi, Marionette)

General

WebDriver BiDi

Marionette

Changes for add-on developers

  • Support provided for page_action as a menus.ContextType in the menus API for Manifest V3 extensions. This provides Manifest V3 extensions the same ability as Manifest V2 extensions to add menu items to page_action. (Firefox bug 1951166)
  • The contextualIdentities API is no longer defined in Firefox for Android. Previously, it was defined but defective. (Firefox bug 1659500)
  • The contextualIdentities permission is now not recognized on Firefox for Android. Previously, it enabled a broken version of the "containers" feature. (Firefox bug 1659500)
  • The new Manifest V3 version of the userScripts API is now available on Firefox for Android. (Firefox bug 1949955)

Removals

Other

Experimental web features

These features are newly shipped in Firefox 138 but are disabled by default. To experiment with them, search for the appropriate preference on the about:config page and set it to true. You can find more such features on the Experimental features page.

Older versions